Clearmg the Compass. 377 



14. Notwithstanding the general agreement of this theory, 

 with many of the observations made in the northern seas, it is 

 still possible that some ships may have no permanent attraction ; 

 and there is reason to believe that the induced magnetism of 

 the iron about a ship may not uncommonly have a perceptible 

 effect on the compass ; especially as it appears, from Mr. Bar- 

 low's experiments, that the guns are to be considered, with 

 respect to magnetism, as soft or conducting . It will there 

 fore be proper to inquire into some of the principal phenomena 

 which may be deduced from this cause. 



15. If all the nails and bolts about the ship, together with the 

 guns and ballast, were equally distributed in all possible 

 directions, with respect to their longest dimensions, or even 

 equally distributed into any three different directions perpendi- 

 cular to each other, the efiect on the needle would be very nearly 

 the same as that of a single bar placed in the direction of the 

 dipping needle, or of a sphere or shell of equivalent dimen- 

 sions ; so that it becomes interesting to inquire what would be 

 the effect of such a sphere on the compass. 



16. Supposing the sphere to be placed immediately before 

 the compass, and on the same level with respect to the decks, 

 the disturbing force would always completely vanish when 

 the ship's head pointed east or west ; so that this is a case 

 which may be excluded from further consideration. 



17. In all other cases it may be shown that the needle, 

 if otherwise at liberty, would be directed towards a point in 

 the magnetic axis of the sphere at which it meets a plane, 

 perpendicular to the line joining the j^pherc and the compass, 



